Runes of Magic US / AU

No, Usually, No

HM is usually bugged af so we run into this sometimes.
Gotta burn fast.

From what I remember some bosses that obtain this buff become invulnerable or near-invulnerable. They either become immune to all your attacks or start healing up to the point where you just stand no chance at all. It really depends on the boss I guess because there are some bosses that you can burn through even with the buff. Some of the bosses just gain insane physical/magical attack/def, but if you burn hard enough and immune or dodge all the damage then it is possible. I remember doing Grotto Hard Mode before and I guess we forgot to pick up the staff from 2nd boss and the body disappeared before we could go back to get it. We had to skip 4th boss and the remaining bosses had the "Earth Destruction" buff, but we were still able to clear the last 3 bosses just fine. As far as any special title there is no such thing and loot is usually just the same.

The buff simply increases the boss attributes, nothing more. The fight still interacts the same, aggro kill boss get loot. Boss heals after fight is so long it activates the rage timer, which is a different scenario entirely.